This invention relates to a four-wheel drive vehicle control apparatus for controlling changeover between two- and four-wheel drive or the locking of a center differential mechanism by sensing the coefficient of friction between a road surface and the tires of the vehicle.
In order to keep a four-wheel drive vehicle in an appropriate traveling state despite changes in the road surface on which the vehicle is running, a four-wheel drive control system has been proposed in which the vehicle is switched over to a control mode different from that of the simple four-wheel drive traveling mode whenever necessary.
Such a control system is of two different types. One is a part-time four-wheel drive system in which, in accordance with the road surface conditions, a changeover can be made whenever necessary between two-wheel drive in which either the front or rear wheels are driven and four-wheel drive in which both the front and rear wheels are driven.
The other type of control system is a full-time four-wheel drive system having a center differential mechanism for absorbing a difference in turning radius that develops between the front and rear wheels at cornering. In order to make it possible to switch the control mode in accordance with the road surface conditions, this system is provided with a locking mechanism for direct transfer between the front and rear wheels without the intervention of the center differential mechanism. The center differential mechanism is locked when a large driving force is necessary, as at acceleration or during travel on a poor road surface, and unlocked during ordinary travel when a large driving force is not required.
In general, control for switching between two- and four-wheel drive in the part-time configuration or for locking the center differential mechanism in the full-time arrangement is a manual operation performed by the driver based on his or her judgment of the road surface conditions. Recently, however, a system has been proposed in which such control is executed automatically by comparing the engine driving force and the force acting upon the wheels of the vehicle. A vital factor for achieving safe and stable vehicle travel with such a system is how to sense the coefficient of friction between the road surface and a tire accurately when calculating the force acting upon the wheel. In the prior art, a sensor is brought into contact with the road surface to sense the coefficient of friction between the road surface and tire so that measures can be taken to deal with the coefficient of friction and road surface condition.
However, with the foregoing conventional method, the state of the road surface on which the vehicle is currently traveling can be judged but not the road surface conditions ahead of the vehicle. Accordingly, since the coefficient of friction between a tire and the road surface lying ahead of the vehicle cannot be sensed, there are cases where the changeover between two-wheel/four-wheel drive or the locking of the center differential mechanism is not controlled appropriately when the conditions of the road surface change. This can lead to tire skidding or lock-up. Thus, in the conventional arrangement, a suitable response to the road surface conditions may be delayed. This can have a detrimental effect upon the safety and stability of vehicle travel.
Furthermore, even if the coefficient of friction between a tire and the road surface ahead of the traveling vehicle could be sensed, the value would be an estimate at best and would not necessarily coincide with the actual coefficient of friction encountered when the particular section of the road is traversed. Tire skidding and lock-up could result unless appropriate control is performed.